It is known that photons belonging to the X-ray band can be counted, i.e. individual X-ray photons can be detected, by making use of a junction between two superposed superconducting electrodes of niobium that are separated by a barrier of Al-AlO.sub.x -Al. Such devices, as described in N. Rando, A. Peacock, A. van Dordrecht, C. Foden, R. Engelhardt, B. G. Taylor, P. Gare, J. Lumley and C. Pereira in Nucl. Instr. Meth. in Phys. Res. A 313 (1992) 173, have low quantum efficiency lying in the range 5% to 25%. Those devices are not at all sensitive to individual photons of lower energy, e.g. visible photons or ultraviolet (UV) photons.
The structure of such devices is briefly outlined with reference to FIG. 1.
Visible or UV photons have been counted, e.g. by means of light amplifiers having microchannels. Such devices have low quantum efficiency and, in addition, they are effective only in relatively narrow passbands.
Photon-counting devices using light amplifiers have low temporal resolution and no spectral resolution.
M. Kurakado, Nucl. Instr. Meth. in Phys. Res. A 314 (1992) 252 and Journal of Applied Physics, Vol. 59, No. 11, pp. 3807-14, describe a superconducting analog optical detector. Unlike a photon-counting device, an analog detector makes it possible to measure effects that result from collective interactions between the sensitive portion of the detector and a large number of optical photons.
A detector of that type can therefore have no temporal resolution concerning the arrivals of various individual photons. Similarly, it is impossible to know the energy of each photon (no spectral resolution).